This invention relates to semiconductive materials, particularly to the conversion of such materials to change their characteristics, and more particularly to the conversion of a limited region of a semiconductive material to permit conduction in solar cells.
It is standard practice to form solar cells by depositing successive patterned layers upon a substrate. Alternatively, the layers are patterned after deposition by removing selected portions to form individual solar cells. The resulting cells are joined externally to form solar arrays. An example is provided by the Borden U.S. Pat. No. 4,278,473. Such arrays are expensive and difficult to manufacture. The patterning processes also reduce the active areas of the cells.
Another type of solar cell is described in the Kuwano et al U.S. Pat. No. 4,281,208 where an unpatterned active layer has electrical interconnections beyond the edge of the layer. This unpatterned structure is easier to manufacture than patterned structures. It is, however, limited in size by the electrode sheet resistance because the interconnection is made at the ends of the cells. As a result, such a panel is impractical for most uses.
Still another type of solar cell is disclosed in Morel et al U.S. Pat. No. 4,517,403 where a continuous semiconductive film carries front and back electrodes. Electrical connection between the electrodes is said to be provided directly through the film itself by diffusion of metal through the film. In particular, Morel uses "stitch bars" on one set of electrodes over which a semiconductive film extends continuously and completely. The stitch bars are tall and rough compared to the thickness of the film. Otherwise the desired conductive passage through the film will not exist. Where the stitch bars are relatively high the application of heat supposedly can cause diffusion of the electrode material, and possibly the material of the stitch bars, into the semiconductive film. The resulting diffused region is allegedly more conductive than the bulk of the film and is said to enhance cell interconnection. The conductive metallic materials allegedly may diffuse into the film as discrete atoms or exist as particulates occupying interstitial sites within the film. It is desirable to apply the heat only at localized areas within preselected areas. A laser beam may be used to melt the contact and the film material to form a eutectic composition of the two materials, or the laser beam may simply heat the materials enough to enhance diffusion.
In practice, heating to produce diffusion does not operate satisfactorily. In particular, heating using a laser beam is incapable of providing the required diffusion. Consequently, in applications of the Morel disclosure it has been necessary to allow the stitch bars of one set of electrodes to extend completely through the semiconductive film into contact with another set of electrodes, since attempted interconnection by diffusion is unworkable.
Accordingly, it is an object of the invention to facilitate the interconnection of electrodes with respect to an intervening semiconductive layer. A related object is to facilitate the production of conductive regions in semiconductive materials.
Still another object of the invention is to overcome the difficulties and limitations associated with the prior art.
Another object is to overcome the difficulties associated with the technique described by Morel where the heating of metallic material is alleged to produce diffusion through an intervening film.